This invention relates to permeability separation devices (permeators) in which the membrane is formed from selectively permeable, hairlike, hollow fiber lengths. These permeators are particularly suited for the recovery of water from brines by reverse osmosis, but specific membranes have the propery of being selectively permeable to different components of fluid mixtures. When pressure is applied to the solution, solvent (permeate) is forced through the membrane while the ions are restrained from passing therethrough. The permeate is separated from the feed and processed feed by potting the ends of the hollow fibers in a resinous wall commonly referred to as a tubesheet and either permitting the ends of the hollow fibers to extend through the tubesheet or cutting the resinous tubesheet and thereby exposing the open ends of the hollow fibers. Such tubesheets are disclosed in McLain U.S. Pat. No. 3,422,008, Clark, U.S. Pat. No. 4,358,377 and Tomsic, U.S. Pat. No. 3,503,515. The problem with such tubesheets is that some structural system must be used to retain the tubesheet in the pressure vessel. One method is to provide a porous backup plate butted against the open ends of the hollow tubes. This construction, however, results in plugging the ends of a great many of the hollow fibers and reducing the efficiency of the permeator. Another system is to enlarge the size of the tubesheet and then provide special couplings to retain the tubesheet. This, however, increases the complexity of the device and raises the cost.
Another method of collecting the permeate from the hollow fibers is disclosed in Clark, U.S. Pat. Nos. 4,061,574 and 4,080,296. In both of these Clark patents bores are drilled along chords into the tubesheet thus eliminating the need for end plates. In U.S. Pat. No. 4,061,574, Clark suggested that the holes might be bored along radial lines, but recognized that many of the hollow fibers would not be intersected and such a pattern of bores would be inefficient. The permeators in both Clark patents required forming an annular groove in the peripheral wall of the tubesheet. This is difficult in practice to achieve since many of the open ends of the hollow fibers are mashed closed in the process of forming the groove. Further, both of the Clark patents call for all the bores to be parallel and along a single chord. It has been difficult in practice to bore holes in the circular tubesheet as the angle between the center line of the bore and a line tangent to the circumference increases. As a result, some of the ends of the hollow fibers are sealed closed in the process of boring holes into the tubesheet and the efficiency of the tubesheet is accordingly decreased.
As the size of the permeators has increased the losses in permeators with uncut fibers has become a matter of practical concern.
The primary object of the present invention is to provide a tubesheet for a permeator in which all of the hollow fibers are free to discharge permeate without restriction.
Another object is to provide an improved tubesheet in which the permeate may drain to either a core within the tubesheet or to a location on the outer periphery of the tubesheet.
A further object is to provide an improved tubesheet in which all of the hollow fibers are intersected with a minimum number of bore holes.
Still another object is to provide a system for drilling bores in which all holes may be drilled at a relatively small angle to the surface of the tubesheet.
A still further object is to provide a tubesheet in which the bores have a relatively small diameter.
A final object is to provide a tubesheet which may be used in a pressure balanced system such as the permeator disclosed in U.S. Pat. No. 4,080,296 or a nonpressure balanced permeator such as the one disclosed in U.S. Pat. No. 4,358,377.